This is a direct reference to Catullus. The poet is showing how only the heart has been leading him up to this point, and as such, he has been 'blinded' by it. Lines 3-4 are a rational evaluation of the situation, as he steps back into a detached state.

in vento... oportet aqua - she ought to write on wind and fast-flowing water

This is a metaphor for her declarations of love. Any writing that was written on wind or water would disintegrate immediately. Therefore, we understand that the poet regards Lesbia's statements as fleeting, said in the moments of passion; in truth they are unreliable and fickle.